FIG. 1A shows an example of a single tone frequency generator. The frequency generator includes an oscillator 2 for producing a signal having the frequency of the single tone, and an output 6. It is known that single tone frequency generators can suffer from the presence of harmonics at the output 6. These harmonics can affect the performance of the generator in terms of its ability to provide a clean signal having a single frequency.
As shown in FIG. 1A, one way of reducing the presence of harmonics is to use a low pass filter 4. The low pass filter 4 is positioned between the oscillator 2 and the output 6.
The effect of the low pass filter 4 is illustrated in FIG. 1B. As shown in FIG. 1B, the low pass filter 4 has an response 8 that allows the first harmonic H1 (which corresponds to the fundamental of the single tone output) to pass, while filtering out the second, third and higher harmonics.
Although the use of a low pass filter of the kind explained above can be effective in removing harmonics that can otherwise affect the performance of a single tone frequency generator, the filter may be expensive. Also, it may be difficult to use a low pass filter for high power applications (≧50 dBm, for example).
In the field of digital data transmissions, it is known to apply a technique that involves modifying a signal prior to amplification, for removing distortion that can otherwise create unwanted emission in adjacent channels. An example of this is explained below in relation to FIG. 2.
FIG. 2 shows a digital data transmission circuit 10. The circuit 10 includes a digital modulator 14 which receives data at an input 12 for transmission by an antenna 20. The circuit 10 also includes an up conversion mixer 16 and a down conversion mixer 24. The circuit 10 further includes a power amplifier 18 and an attenuator 22. The down conversion mixer 24 and the attenuator 22 are provided in a feedback circuit that is connected to the output of the power amplifier 18 by a coupler 19.
During operation, the up conversion mixer 16 provides up conversion in the desired channel for digital data transmission and this up converted signal is provided then to the power amplifier 18 and subsequently transmitted via the antenna 20. Part of the output signal is fed back via the coupler 19, through the attenuator 22 and is down converted by the down conversion mixer 24. This feedback signal is then provided to the digital modulator 14. Complex algorithms are then used in the digital modulator to minimise the power in the unwanted channels. In essence, the digital modulator 14 modifies the data input 12 by applying predistortion to the signal in accordance with the feedback in order to minimise unwanted emissions in side channels of the transmitted signal. This is illustrated in the graph in the lower part of FIG. 2, which illustrates that the adjacent channel power ratio (ACPR) of the wanted digital channel 26 compared to the intermodulation signals in the unwanted side channels 28 can be enhanced using the circuit 10.
The circuit shown in FIG. 2 is used, as noted above, for digital data transmission and as such the predistortion introduced by the digital modulator 14 is relevant only in the case of a modulated signal. Additionally, the circuit 10 shown in FIG. 2 requires a large bandwidth in order to be able to detect the power in the adjacent channels 28. Accordingly, the circuit 10 shown in FIG. 2 is not useful in the case of single tone applications.
One example of a single tone application is a RF heating apparatus. This is shown schematically in FIG. 3. In FIG. 3, the RF heating apparatus includes an oscillator 2 which produces a signal which is amplified by a power amplifier 5. The amplified signal is then introduced into a cavity 30 of the RF heating apparatus using an antenna 20. The cavity 30 itself can act as a low pass filter and can thus provide at least some attenuation of the unwanted harmonics of a single tone signal. Nevertheless, there exists a need for the production of a single tone RF signal having a low level of harmonics.
As noted above, a low pass filter may be expensive and/or difficult to use for high power applications such as RF heating. Because of this, it may generally be necessary to place the low pass filter used in an RF heating apparatus in front the power amplifier. This is illustrated in FIG. 4.
The circuit of FIG. 4 includes an oscillator 2, a power amplifier 5 and a low pass filter 4 located in between the oscillator 2 and the power amplifier 5. Note that the circuit in FIG. 4 is similar that that described above in relation to FIG. 3, and may be used for RF heating applications. A problem with the arrangement shown in FIG. 4 however, is that the power amplifier 5 itself can introduce unwanted harmonics into the single tone signal. Because the low pass filter 4 is placed before the power amplifier, it cannot remove harmonies that are introduced by the power amplifier. Accordingly, the low pass filer 4 only has limited effect in reducing the level of harmonics at the output.
An article by M. Abid et al. entitled “Mixed Cartesian Feedback for Zero-IF WCDMA Transmitter”, IEEE, 2011, describes an adaptive power amplifier (PA) linearization technique. A WCDMA zero-intermediate frequency (Zero-IF) transmitter is provided with a modified Cartesian feedback (CFB) loop. The transmitter architecture consists of an analogue stage including forward I/Q modulator and feedback I/Q demodulator, and a digital stage adjusting the phase rotation around the loop.